Chlorine isotopes unravel conditions of formation of the Neoproterozoic rock salts from the Salt Range Formation, Pakistan

During the late Neoproterozoic, the Salt Range in Pakistan was one of the regions where the Tethys truncated and marine strata developed. The numerous transgressions and regressions that occurred during that period provided enough initial material for the development of marine evaporites. The geology of the Salt Range is characterized by the presence of dense salt layers and the existence of four regional and local scale unconformities. These thick salt deposits geologically favor potash formation. Here we coupled chloride isotope geochemistry and classical chemistry of local halite samples to assess the extent of brine evaporation that ultimately formed the salt deposits. Our results indicate that evaporites in the Salt Range area are Br-rich and precipitated from seawater under arid climate conditions. The corresponding delta Cl-37 values vary from -1.04 parts per thousand to 1.07 parts per thousand, with an average of -0.25 parts per thousand +/- 0.52 parts per thousand, consistent with the isotope range values reported for other evaporites worldwide. The positive delta Cl-37 values we obtained indicate the addition of nonmarine Cl, possibly from reworking of older evaporites, the influx of dilute seawater, the mixing of meteoric and seawater, and the influence of gypsum-dehydration water. The negative Cl isotope compositions (delta Cl-37 < -1 parts per thousand) indicate that brines reached the last stages of salt deposition during the late Neoproterozoic. We conclude that the Salt Range Formation could be promising for K-Mg salts.